This proposal requests funding for acquisition of a new, modern continuous wave (CW) X-band (~9 GHz) electron paramagnetic resonance (EPR) spectrometer, Bruker Elexsys E500 equipped with E560D electron-nuclear double resonance (ENDOR) system, to support the NIH-funded biochemical/biomedical research at the University of Arizona. This spectrometer will be integrated into the existing EPR Facility at the Department of Chemistry and Biochemistry of the University of Arizona, where it will replace the existing aging (~20 years old) Bruker ESP300E CW EPR instrument. The old spectrometer has developed hardware problems, many of its components are completely outdated, and there is a high probability of a critical failure at any time. Such a failure will impede the progress of the projects of the NIH-funded users relying on EPR in their research. The replacement of the old spectrometer by a new, modern one will ensure an uninterrupted access of the users to EPR and will enrich their research capabilities through a significantly (up to an order of magnitude) higher sensitivity of the new instrument and through providing access to CW ENDOR experiments. In short term, this spectrometer will be used in the ongoing biochemical/biomedical research projects of the users who contributed to this proposal. In the long term, this spectrometer will answer the need in EPR experiments of the future NIH-funded researchers and other users of the EPR Facility at the University of Arizona. The specific affected biochemical/biomedical projects described in this proposal include: (1) Using EPR and CW ENDOR to study the molybdenum catalytic center of sulfite-oxidizing enzymes and model synthetic Mo(V) complexes by Dr. J.H. Enemark. This research is important because some point mutations of human sulfite oxidase has fatal consequences;(2) Using EPR for understanding structural and dynamic aspects of chemistry of nitric oxide, NO, which has numerous biological functions important for human health by Drs. F.A. Walker, K.M. Miranda and W.R. Montfort;(3) Using EPR to test the efficiency of newly developed potential cancer drugs suppressing the function of ribonucleotide reductase by Dr. R.T.Dorr;(4) Using EPR to study Cu binding to proteins in relation to metal homeostasis and metal resistance by Dr. M. McEvoy.